1. Field of the Invention
The invention relates to multilayer printed circuit boards and multichip-module substrates having at least one layered composite of high-temperature-stable plastic and conductive tracks, as well as to a method for their manufacture.
2. Description of Related Art
Multichip modules (MCM) are components which connect integrated circuits, such as memory and logic modules, to one another and to the peripheral units. The resulting shortened current paths permit shorter signal propagation delays and a higher clock frequency. The multichip modules are manufactured using multilayer technology, ceramic or silicon being suitable as substrate material. Also suitable as substrates are printed-circuit boards, which--in the same way as ceramic materials--can have a multilayer structure. The actual microcircuit wiring on the substrate (up to 25 conductor wiring layers and more) consists of copper conductors, which are insulated from one another by an organic dielectric material, such as polyimide. Via holes from one copper conductor layer to the next electrically connect the individual copper layers (see: "IEEE Spectrum", vol. 27 (1990), no. 3, pp. 34-36, 46 and 48; "The International Journal for Hybrid Microelectronics", vol. 13 (1990), no. 4, pp. 91-99; "VDI-Berichte" ["VDI Reports"], no. 933 (1991), pp. 265-283).
Due to their low dielectric constant, organic dielectric materials permit higher clock frequencies than ceramic insulation layers; however, higher clock frequencies will be required in the future more and more. High-temperature-stable organic polymers, such as polyimides and polybenzocyclobutenes, are primarily used, since they largely satisfy the electrical requirements (low dielectric constant, low loss angle) and the thermal requirements (stability during soldering processes).
High-temperature-stable organic dielectric materials, such as polyimides, have, however, the property of absorbing up to 4 mass % of water from the ambient humidity (see A. Hammerschmidt et al., 1st European Technical Symposium on Polyimides, May 10-11, 1989, Montpellier, France). As a consequence, the electrical properties, particularly the dielectric constant, are adversely affected. This, in turn, exacerbates the switching performance (clock frequency) of the entire subassembly. Furthermore, the moisture can initiate or accelerate corrosion processes inside the multichip modules.
At the present time, moisture barriers are not being applied to multichip modules having organic dielectric materials. A moisture barrier is not absolutely necessary because of encapsulation, packaging in air-conditioned areas, and layer thicknesses of up to 60 .mu.m per layer. A protection of this type is actually necessary, however, for new design concepts with dielectric layer thicknesses of 15 to 20 .mu.m per layer, as well as for low-cost applications.
In the case of integrated circuits formed in multilayer construction with organic dielectric materials (up to three conductor planes; the layer thickness of the dielectric material being from 1 to 2 .mu.m)--in order to suppress dielectric fluctuations caused by variations in moisture--the planarizing dielectric materials are covered with a layer of silicon nitride, so-called plasma nitride, which acts as a moisture barrier (see: Fachtagung "Polymerwerkstoffe fur die Elektronik" [Technical Conference "Polymer Materials for Electronics"], Oct. 14-16, 1987, Wurzburg, Conference Report (1987), pp. 153-165). However, silicon nitride has the disadvantage of being brittle even at layer thicknesses below 0.25 .mu.m. Moreover, a different etching process (fluoro etching) must be applied for the requisite structuring of the silicon nitride (to expose contact pads) than is used to structure the subjacent organic dielectric material, such as polyimide (O.sub.2 /RIE or wet etching).
To avoid the disadvantages associated with silicon nitride, it is already known to use amorphous, hydrogenated carbon as a coating in the case of integrated circuits (see European patent application Ser. No. 0 381 109 A2). In so doing, however, unfavorable effects can result from compressive stress.